1. Field of the Invention
The present invention relates to a system and method for controlling intake air to an internal combustion engine by variable valve timing.
2. Terminology
Engine Displacement
This is used herein to mean the sum of displacements of all of cylinders of an engine. “Engine displacement” and “displacement of an engine” are synonyms. In the description, the reference character “VOL#” is used to represent the “engine displacement”.
3. Description of Related Art
JP-A 8-200025 discloses a system for controlling electromagnetic drivers (EMDs) for intake and exhaust valves provided per each combustion chamber of an internal combustion engine. According to this system, an electronic control unit (ECU) controls valve timings of intake and exhaust valves independently from the crankshaft position and speed in one of predetermined schedules fit for various engine operation ranges. The ECU identifies which one of the various engine operation ranges against varying engine load and engine speed. During transient period from one to another of the engine operation ranges, the ECU shifts the valve timings of intake valves in a gradual manner so as to prevent occurrence of a rapid change in intake air characteristic. This JP-A is silent as to how engine load is controlled and has no teaching with regard to engine load control by early or delayed valve closure timings of the intake valves.
The present invention aims at controlling intake air by varying valve closure timings of intake valves using such EMDs. The electromagnetic drivers can adjust the valve opening and closure timings over a wide range independently from the crankshaft position. Since throttling of intake air is not relied upon to control the engine load, the engine pumping losses are eliminated.
Because of the provision of an intake manifold and an intake collector downstream of a throttle valve, there is a delay, during throttled intake air control, between a change in angular position of the throttle valve and a change in cylinder air charge caused by the change in throttle angular position. In the case of unthrottled intake air control, there is no delay caused due to the intake manifold and intake collector so that a change in valve closure timing induces a change in cylinder air charge without any delay, providing aggressive response performance. Thus, operator aggressive cyclic depression and release of accelerator pedal induces violent torque change imparted to a power train induces vibration, providing a reduction in ride comfort and elevated noise level.
Unthrottling intake air control is satisfactory. However, it cannot control cylinder air charge satisfactorily in a certain operation range. In such operation range, throttling of intake air by a throttle valve is needed. In such case, care must be taken to provide a smooth transition during the transient period from the unthrottled control to the throttled control or vice versa without any shock due to a torque change. Undesired torque change might take place during such transient period due mainly to a considerable difference in response performance between the two controls.
Thus, a need remains to improve an unthrottled intake air control by variable valve timing such that the occurrence of violent torque variation caused by aggressive cyclic depression and release manipulation of an accelerator pedal is prevented and the undesired torque change during transient period from one to the other of the two intake air controls is suppressed.